How Glaciers Work

This diagram shows a small mountain glacier as it might look if we could cut it
down the middle and see its interior.

Labeled Diagram

Glaciers form when it becomes cold enough that all the snow that falls each
winter cannot melt during the summer. It may be cold enough for ice to
accumulate either at high altitudes or near the poles. In mountains, like shown
here, there is more snow in the winter at high elevations than can melt in the
summer. In the zone of accumulation, ice builds up until it becomes thick
enough to flow. It flows downhill into warmer elevations. Here, in the zone
of ablation, it is warm enough to melt all the winter snow, but ice flowing
down from above makes up the difference. When melting balances ice flow, the
glacier ends. Changes in ice flow or climate may cause the ice front to advance
or retreat until the balance is re-established. The boundary between the zone of
accumulation and the zone of ablation is called the firn line and is
often a sharp boundary where white recent snow above the firn line meets older,
darker ice below it.

As snowfall varies with the seasons, the flow of ice speeds up and slows
down. Many glaciers have curving ridges, called ogives, that mark
alternating periods of fast and slow flow, much like the growth rings in a tree.

Ice near the surface in the brittle zone breaks under stress, but
deeper down where the pressure is greater, the ice flows without breaking. This
zone is called the ductile zone. If the glacier flows over an
obstruction, the brittle surface breaks up into crevasses.

Since ice flows like a very thick fluid, it can't get into tight spots and
erode like water can. Instead it simply planes off whatever is in the way,
creating the U-shape so typical of glacial valleys. As the ice flows down from
its source, it plucks rock away from the head of the valley, creating a
bowl-shaped basin called a cirque. Some of the debris carried by the
glacier is deposited along the sides to form a lateral moraine. Debris dumped at the end of the
glacier is called a terminal moraine.

Melt water flows through tunnels in the ice and may emerge at the end as a
melt water stream. Beneath the glacier, the water may be under hundreds of feet
of ice and is under great pressure. If it can escape, the water pressure plus
the sediment in the water can carve rock like a saw. If there are large amounts
of melt water under the glacier, it lubricates the ice and may allow the glacier
to advance very quickly.